1. Field of the Invention
The present invention relates to reducing the level of noise in a speech signal.
2. Background Art
Electrical renditions of human speech are increasingly used for inter-person communication, storing speech and for man-machine interfaces. One limit on the comprehensibility of speech signals is the amount of noise intermixed with the speech. A wide variety of techniques have been proposed to reduce the amount of noise contained in speech signals. Many of these techniques are not practical because they assume information not readily available such as the noise characteristics, location of noise sources, precise speech characteristics, and the like.
One technique for reducing noise is to filter the noisy speech signal. This may be accomplished by converting the speech signal into its frequency domain equivalent, multiplying the frequency domain signal by the desired filter then converting back to a time domain signal. Converting between time domain and frequency domain representations is commonly accomplished using a fast Fourier transform and an inverse fast Fourier transform. Alternatively, the speech signal may be broken into subbands and a gain applied to each subband. The amplified or attenuated subbands are then combined to produce the filtered speech signal. In either case, filter or gain parameters must be calculated. This calculation depends upon determining characteristics of noise contaminating the speech signal.
Typically, speech contains quiet periods when only the noise component appears in the speech signal. Quiet periods occur naturally when the speaker pauses or takes a breath. A voice activity detector (VAD) may be used to detect the presence of speech in a speech signal. In use, a VAD is connected to the noisy speech signal. The output of the VAD signals parameter calculation logic when speech is occurring in the input signal. One problem with using a VAD is that the VAD is typically complex if the speech signal contains widely varying levels of noise.
What is needed is to produce improved speech signals in the presence of varying levels of noise without requiring complex logic for calculating noise reducing coefficients.